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it is fb, but there's a UK 'buy, sell, trade drums' group which is decent enough, got a double-pedal and a cymbal from a couple of different people on there in the past few years without any issues. (edit: i used this one https://www.facebook.com/groups/507827252616774 - there's another version that's not '(2)', not sure what the difference is)
also bought lots of used stuff here: https://www.grahamrusselldrums.com/pre-owned/used-drum-hardware/ though can't see the pedals you're after in stock at the mo.
Any suggestions for places online to look for secondhand gear? Other than eBay/Facebook. Even better, does anyone want to sell me a Pearl Eliminator Redline double pedal?